The Binding of Monovalent Electrolyte Ions on α-Alumina. I. Electroacoustic Studies at High Electrolyte Concentrations

An electroacoustic technique has been used to monitor the binding of various monovalent inorganic ions on α-alumina at different pHs, nondilute solids, and moderate to high (0.01−1.0 mol dm - 3 ) electrolyte concentrations. The ζ potential versus pH data show that NaNO 3, KNO 3, CsNO 3, KBr, KCl, and KI are indifferent electrolytes for the α-alumina surface. However, LiNO 3 causes a significant change in the isoelectric point of α-alumina, indicating that Li + adsorbs in a specific manner to the surface over the range of concentrations investigated. At high (1.0 mol dm - 3 ) electrolyte concentrations, the monovalent cations bind to the negative α-alumina surface in the order Li + > Na + > K + ≈ Cs + . By contrast, the Br -, Cl -, I -, and NO 3 - anions adsorb to an almost identical extent over the entire range of concentrations and pH conditions investigated. The cation binding sequence is consistent with the water “structure making−structure breaking” model first proposed by Gierst et al. 1 and Bérubé and de Bruyn, 2 which is based on the hydration enthalpies of the ions and the heat of immersion of the colloidal substrate. The comparable anion adsorption behavior is believed to arise because of the similar and/or low hydration enthalpies of the anions, which lead to a similar anion−surface interaction in each case.